Answer
b) Equilibrium Length = 15
c) Shortest length = 11.5
d) Longest Length = 18.5
Work Step by Step
Length of spring at any time is given by
$L = 15 - 3.5 \cos (2\pi t)$
b) Equilibrium Length
It is the original length of the spring = 15, when $\cos (2\pi t) = 0$
$- 3.5 \cos (2\pi t)$ is the oscillation part
c) Shortest length
L_shortest = 15-3.5 = 11.5, when $\cos (2\pi t) = 1$
d) Longest Length
L_longest = 15 + 3.5 = 18.5 when $\cos (2\pi t) = -1$
